RMI原理揭秘之远程对象

转载：http://guojuanjun.blog.51cto.com/277646/1423392/

 

1. 定义远程接口:

2. package  com.guojje;
   import  java.rmi.Remote;
   import  java.rmi.RemoteException;
   public  interface  IHello  extends  Remote {
       public  int  helloWorld() throws  RemoteException;
   }

 3. 定义实现类：

package  com.guojje;
import  java.rmi.RemoteException;
import  java.rmi.server.UnicastRemoteObject;

public  class  Hello  extends  UnicastRemoteObject  implements  IHello {
    private  static  final  long  serialVersionUID = 1L;
    private  int  index =  0 ;
    protected  Hello()  throws  RemoteException {
    }
    @Override
    public  int  helloWorld(){
        System.out.println( "Hello!" );
        return  ++index;
    }
}

 4.服务端:

package  com.guojje;
import  java.rmi.registry.LocateRegistry;
import  java.rmi.registry.Registry;

public  class  HelloServer {
    public  static  void  main(String args[]) {
        try  {
            IHello rhello =  new  Hello();
            Registry registry = LocateRegistry.createRegistry( 8888 );
            registry.bind( "test" , rhello);
            System.out.println( "Remote Hello Object is bound sucessfully！" );
        }  catch  (Exception e) {
            e.printStackTrace();
        }
    }
}

5.客户端:

package  com.guojje;
import  java.rmi.Naming;
public  class  HelloClient {
    public  static  void  main(String args[]) {
        try  {
            for  ( int  i =  0 ; i <  5 ; i++) {
                IHello rhello = (IHello) Naming
                        .lookup( "rmi://localhost:8888/test" );
                System.out.println(rhello.helloWorld());
            }
        }  catch  (Exception e) {
            e.printStackTrace();
        }
    }
}

6.输出结果：  

1)服务端输出:

 Remote Hello Object is bound sucessfully！
Hello!
Hello!
Hello!
Hello!
Hello!

2)客户端输出：

0
1
2
3
4

7.把实现类更改为不继续承UnicastRemoteObject

package  com.guojje;
import  java.io.Serializable;
import  java.rmi.RemoteException;

public  class  Hello  implements  IHello,Serializable {
    private  static  final  long  serialVersionUID = 1L;
    private  int  index =  0 ;
    protected  Hello()  throws  RemoteException {
    }
    @Override
    public  int  helloWorld(){
        System.out.println( "Hello!" );
        return  ++index;
    }
}

8.输出结果：  

1)服务端输出:

 Remote Hello Object is bound sucessfully！

2)客户端输出：

Hello!
1
Hello!
1
Hello!
1
Hello!
1
Hello!
1

这两个用例的输出结果来看，前一个用例index计数器一直在增加,且Hello!输出在服务端。这说明

helloWorld方法执行是在服务端，客户端的每一次对象方法调用，都是对服务端对象的调用。

而后一个用例就不同了。helloWorld方法执行是在客户端,且每次lookup出来的都是新的对象。

我们看一下lookup出来的对象类型：

package  com.guojje;
import  java.rmi.Naming;
public  class  HelloClient {
    public  static  void  main(String args[]) {
        try  {
            for  ( int  i =  0 ; i <  5 ; i++) {
                IHello rhello = (IHello) Naming
                        .lookup( "rmi://localhost:8888/test" );
                System.out.println(rhello.getClass());
                System.out.println(rhello.helloWorld());
            }
        }  catch  (Exception e) {
            e.printStackTrace();
        }
    }
}

实现类继承UnicastRemoteObject时，lookup出来的对象类型是$Proxy0，而不继承UnicastRemoteObject时类，对象类型是com.guojje.Hello。

我们把继承UnicastRemoteObject类的对象叫做远程对象，我们lookup出来的对象，只是该远程对象的存根(Stub)对象,而远程对象永远在远方。客户端每一次的方法调用，最后都是仅有的那一个远程对象的方法调用。

没有继UnicastRemoteObject类的对象，同样可以bind到Registry，但lookup出来了对象却是远程对象

经过序列化，然后到客户端反序列化出来的新的对象，后续的方法调用与远程对象再无关系。

 

 

以下是理论部分：

那UnicastRemoteObject类的继承是如何影响这些的呢? 我们来探索一下。

package  com.guojje;
public  class  HelloServer {
    public  static  void  main(String args[]) {
        try  {
            new  Hello();
        }  catch  (Exception e) {
            e.printStackTrace();
        }
    }
}

仅仅new一个远程对象，运行这个程序，我们就发现进程不会退出。它hang在哪儿呢？

jstack查看线程栈，发现有SocketAccept在监听：

的确启动了一个监听端口。ServerSocket类上添加debug.得到调用栈如下：

UnicastRemoteObject基类的构造方法将远程对象发布到一个随机端口上，当然端口也可以指定。

 protected  UnicastRemoteObject( int  port)  throws  RemoteException
    {
        this .port = port;
        exportObject((Remote)  this , port);
    }

  public  static  Remote exportObject(Remote obj,  int  port)
        throws  RemoteException
    {
        return  exportObject(obj,  new  UnicastServerRef(port));
    }

 /**
     * Exports the specified object using the specified server ref.
     */
    private  static  Remote exportObject(Remote obj, UnicastServerRef sref)
        throws  RemoteException
    {
        // if obj extends UnicastRemoteObject, set its ref.
        if  (obj  instanceof  UnicastRemoteObject) {
            ((UnicastRemoteObject) obj).ref = sref;
        }
        return  sref.exportObject(obj,  null ,  false );
    }

迎来一个重要的方法(UnicastServerRef.java)：

 /**
     * Export this object, create the skeleton and stubs for this
     * dispatcher.  Create a stub based on the type of the impl,
     * initialize it with the appropriate remote reference. Create the
     * target defined by the impl, dispatcher (this) and stub.
     * Export that target via the Ref.
     */
    public  Remote exportObject(Remote impl, Object data,
                               boolean  permanent)
        throws  RemoteException
    {
        Class implClass = impl.getClass();
        Remote stub;

        try  {
            stub = Util.createProxy(implClass, getClientRef(), forceStubUse);
        }  catch  (IllegalArgumentException e) {
            throw  new  ExportException(
                "remote object implements illegal remote interface" , e);
        }
        if  (stub  instanceof  RemoteStub) {
            setSkeleton(impl);
        }

        Target target =
            new  Target(impl,  this , stub, ref.getObjID(), permanent);
        ref.exportObject(target);
        hashToMethod_Map = hashToMethod_Maps.get(implClass);
        return  stub;
    }

这里的stub变量就是我们lookup出来的远程对象的存根对象。而target保留了远程对象的信息集合：

对象，存根，objId等。

接着看TCPTransport.java的exportObject方法：

  /**
     * Export the object so that it can accept incoming calls.
     */
    public  void  exportObject(Target target)  throws  RemoteException {
        /*
         * Ensure that a server socket is listening, and count this
         * export while synchronized to prevent the server socket from
         * being closed due to concurrent unexports.
         */
        synchronized (this) {
            listen();
            exportCount++;
        }

        /*
         * Try to add the Target to the exported object table; keep
         * counting this export (to keep server socket open) only if
         * that succeeds.
         */
        boolean  ok =  false ;
        try  {
            super .exportObject(target);
            ok =  true ;
        }  finally  {
            if  (!ok) {
                synchronized  ( this ) {
                    decrementExportCount();
                }
            }
        }
    }

listen()启动监听。这里对TcpTransport加了同步，防止多个线程同时执行,同时也防止同一端口启动多次。

一次TcpTransport会有一个监听端口，而一个端口上可能会发部多个远程对象。exportCount计录该TcpTransport发布了多少个对象。

super .exportObject(target);

/**
     * Export the object so that it can accept incoming calls.
     */
    public  void  exportObject(Target target)  throws  RemoteException {
        target.setExportedTransport( this );
        ObjectTable.putTarget(target);
    }

ObjectTable为静态方法调用，那么所有的远程对象信息(target)都会放到这个对象表中。我们这个target包含了远程对象几乎所有的信息，找到他，就找到远程对象的存根对象。

远程对象的发布，先说这么多。我们再看一下lookup是如何找到远程对象的存根的.

当然先从远程对象的bind说起:

RegistryImpl.java:

 /**
     * Binds the name to the specified remote object.
     * @exception RemoteException If remote operation failed.
     * @exception AlreadyBoundException If name is already bound.
     */
    public  void  bind(String name, Remote obj)
        throws  RemoteException, AlreadyBoundException, AccessException
    {
        checkAccess( "Registry.bind" );
        synchronized  (bindings) {
            Remote curr = bindings.get(name);
            if  (curr !=  null )
                throw  new  AlreadyBoundException(name);
            bindings.put(name, obj);
        }
    }

bindings里放得确实是远程对象本身，而不是他的存根。这是怎么回事？继续追究lookup方法。

RegistryImpl_Skel类是rmic生成所有没有源代码，我们只能从反编译的代码中查看一点信息：

case  2 :  // lookup(String)
    {
        java.lang.String $param_String_1;
        try  {
        java.io.ObjectInput in = call.getInputStream();
        $param_String_1 = (java.lang.String) in.readObject();
        }  catch  (java.io.IOException e) {
        throw  new  java.rmi.UnmarshalException( "error unmarshalling arguments" , e);
        }  catch  (java.lang.ClassNotFoundException e) {
        throw  new  java.rmi.UnmarshalException( "error unmarshalling arguments" , e);
        }  finally  {
        call.releaseInputStream();
        }
        java.rmi.Remote $result = server.lookup($param_String_1);
        try  {
        java.io.ObjectOutput out = call.getResultStream( true );
        out.writeObject($result);
        }  catch  (java.io.IOException e) {
        throw  new  java.rmi.MarshalException( "error marshalling return" , e);
        }
        break ;
    }

从网络流中读取第一个参数必须是lookup的字符串参数。然后调用服务端RegistryImpl对象lookup

方法，该方法返回的的确是远程对象，而非远程对象的存根呀？

问题的原因在于下面的序列化过程，继续看调用栈：

看一个重要方法MarshalOutputStream.java

/**
     * Checks for objects that are instances of java.rmi.Remote
     * that need to be serialized as proxy objects.
     */
    protected  final  Object replaceObject(Object obj)  throws  IOException {
        if  ((obj  instanceof  Remote) && !(obj  instanceof  RemoteStub)) {
            Target target = ObjectTable.getTarget((Remote) obj);
            if  (target !=  null ) {
                return  target.getStub();
            }
        }
        return  obj;
    }

如果对象是java.rmi.Remote实例，则向对象表中找到该对象的Target,如果存在，则返回其存根对象。

所以返回服务端的变成了远程对象的存根。先到此，后续再探索其方法调用，安全等相关问题。

 

补充：

其实Registry的实现类RegistryImpl也是个远程对象，这里registry.bind却没有进行远程调用。这是因为我是用LocateRegistry.createRegistry(8888)创建的远程对象，而不是通过LocateRegistry.getRegistry(8888)获取的远程对象,而getRegistry返回的是RegistryImpl的Stub.仅此而已。